The invention relates to arc interrupters.
To assist in extinguishing arcs in arc interrupters, which operate in an insulating fluid, for example sulphur hexafluroide (SF.sub.6) gas under pressure, it has been proposed to rotate the arc about an axis. Examples of such arc interrupters are described in UK Patent Applications Publication Nos. 2038100A, 2044538A, 2052160A and 2092385A and in UK Patent No. 2119573B (equivalent to U.S. Pat. No. 4,503,302). Typically, such arc interrupters have fixed and movable contacts, an annular arcing electrode and an arc-driving coil connected in series with the electrode. In such interrupters, the arc, once fully established, extends between the electrode and one of the contacts and is driven by the coil about the central axis of the electrode.
To avoid problems associated with insulating the arc-driving coil from the arc, it is preferred to locate the coil externally of the electrode.
However, as the rating of the interrupters (for example, as described in UK Patent No. 2119573B) are increased, it was found that plain cylindrical electrodes tended to collapse owing to the increased magnetic fields generated by the currents induced in the electrode by the magnetic fields generated by the coil, which magnetic fields are in opposition to one another.
Although the problem can be alleviated to some extent by increasing the cross-section of the electrode at lower ratings, it is not a practical solution at higher ratings owing to the increased costs of material and increased weight and size of the electrode.
It is possible to increase the strength of the electrode by providing radially-extending flanges. Some such electrode configurations are shown in the above-mentioned published Applications. However, in those instances, the flanges appear to have been provided for other reasons, for example to provide an arc runner or to protect the coil ends from the arc (which is initally struck in the immediate vicinity of the coil ends).
It has been found, however, that the electrode can distort owing to the generation in the flanges of magnetic fields, which oppose the fields generated by the coil, by currents induced in the electrode by the fields generated by the coil.